1. Field of the Invention
The present invention relates to an FSK (frequency shift keying) receiver for receiving an FSK-modulated signal and demodulating it into a baseband signal and, in particular to an automatic frequency control technique for use in the FSK receiver.
2. Description of the Prior Art
In general, there are two types of FSK receivers: superheterodyne type and direct-conversion type. They are both provided with a frequency converter and a frequency-to-voltage (f/V) converter. The frequency converter mixes a received FSK signal to a local oscillation signal of a local oscillator. Thereby, the received FSK signal is converted to a second FSK signal of an intermediate frequency. Thereafter, the frequency of the second FSK signal is converted into a voltage that varies according to a change in frequency of the second FSK signal. In general, an f/V converter has a conversion characteristic such that the output voltage increase as the frequency of an FSK signal increases and decreases as it decreases (see FIG. 5). Therefore, the f/V converter can be used to demodulate the FSK signal to produce a baseband signal.
In such an FSK receiver, a frequency drift occurring in a local oscillator can be one of factors that deteriorate receiving status conditions. The frequency drift may be caused by a change in accuracy and/or temperature of the local oscillator. Therefore, an auto frequency control (AFC) technique is employed to cause the local-oscillation frequency to pull in a proper frequency.
A conventional AFC circuit will be described hereinafter with these conventional superheterodyne and direct-conversion receivers having the f/V conversion function.
In a superheterodyne FSK receiver, the output voltage of the f/V converter is input to an integrator where it is averaged. The average is input to a voltage comparator, which compares it to a reference voltage. Then, when the output voltage of the integrator is higher than the reference voltage as the result of the comparison, the voltage comparator raises the local-oscillation frequency of the local oscillator so that the output voltage of the integrator becomes equal to the reference voltage. On the other hand, when the output voltage of the integrator is lower than the reference voltage, the voltage comparator lowers the local-oscillation frequency of the local oscillator so that the output voltage of the integrator becomes equal to the reference voltage. In this case, the reference voltage is a voltage corresponding to the center frequency of the second FSK signal obtained by the frequency converter. In this manner, the conventional AFC circuit uses the integrator and a voltage comparator to perform the automatic frequency control.
Such a conventional AFC circuit can be also applied to a direct-conversion FSK receiver, an example of which has been disclosed in Japanese Patent application Laid-open No. 08-107428. This direct-conversion FSK receiver is provided with a first local oscillator and a second local oscillator. The first local oscillator is used to directly convert the radio-frequency FSK signal into baseband I and Q signals. The second local oscillator is used to up-convert the baseband I and Q signals into an intermediate-frequency signal. Such a system was proposed by WEAVER et al. (Proceedings of The IRE, Jun. 25, 1956, p. 1703-).
The output signal of the second local oscillator is input to a first f/V converter and the intermediate-frequency signal is input to a second f/V converter. The first output voltage of the first f/V converter and the second output voltage of the second f/V converter are compared by a voltage comparator. The output of the voltage comparator is averaged and then the averaged voltage is used to control the frequency of the first local oscillator.
Another conventional circuit has been disclosed in Japanese Utility Model Application Laid-Open No. 61-15816. This conventional circuit is provided with a phase and frequency comparator, which outputs two signals to two detectors through two low-pass filters and then two high-pass filters. respectively. The frequency can be changed by changing a time constant of at least one of the high-pass filters.
The above prior arts for performing automatic frequency control by integrating (averaging) an f/V-converted output signal have the disadvantages described below.
To properly operate the integrator or the averaging circuit, received data must uniformly alternate the signal peaks shown in FIG. 1A as 1's and 0's. When receiving a signal with alternating 1 and 0 non-uniformly as shown in FIG. 1B such as "101011110 . . . ", the integrator or the averaging circuit outputs an erroneous control voltage as shown by the broken line DL in FIG. 1B, resulting in an non accurate local-oscillation frequency. Therefore, it is necessary to operate the automatic frequency control circuit when receiving a uniformly 1 and 0 alternating signal as shown in FIG. 1A.
Moreover, the integrator or the averaging circuit requires an integration or averaging time longer than the data rate. Particularly, a sync signal tends to be short due to recent increase of data transmission rates. Therefore, the conventional automatic frequency control circuit using the integrator or the averaging circuit has a problem that it is difficult to accurately set a local-oscillation frequency for one-time AFC operation.